The Reds reassigned Mathias on Wednesday.
Mathias (shoulder) will start at third base and bat sixth in Monday's split-squad Cactus League game against the Brewers.
Mathias (shoulder) signed a minor-league contract with the Reds on Nov. 29.
Mathias (shoulder) elected to become a free agent Friday.
The Giants sent Mathias (shoulder) outright to Triple-A Sacramento on Wednesday.
San Francisco transferred Mathis (shoulder) from the 10-day injured list to the 60-day injured list Tuesday.
The optional assignment of Mathias was reversed and he has been placed on the 10-day injured list Friday with a right shoulder strain, Susan Slusser of the San Francisco Chronicle reports.
The Giants optioned Mathias to Triple-A Sacramento on Monday, Alex Pavlovic of NBC Sports Bay Area reports.
Mathias was recalled from Triple-A Sacramento on Sunday.
Mathias was traded from the Mariners to the Giants on Monday along with AJ Pollock for an undisclosed return, Jeff Passan of ESPN.com reports.